1. Field of the Invention
The present invention relates to a valve device for circulating and interrupting a fluid, and more particularly, to a paper sheet pickup apparatus for picking up sheets of paper one by one by attaching each sheet to a belt using a suction force.
2. Description of the Related Art
There is a conventional standard paper sheet pickup apparatus, in which a belt with holes runs along mail matters to attach thereon each mail matter by drawing each mail matter through the holes of the belt using a suction nozzle provided near the reverse side of the belt, thereby picking up the mail matters one by one (see, for example, U.S. Pat. No. 5,391,051). This apparatus comprises a solenoid valve interposed between the suction nozzle and a vacuum tank.
When operating the apparatus to pick up mail matters, the belt runs, and each mail matter is attached to the belt by the suction force of the suction nozzle that is generated when the solenoid valve is open. To sequentially pick up mail matters, the solenoid valve is periodically closed in accordance with the pickup timing of each mail matter, thereby forming a gap between a currently picked up mail matter and a mail matter to be subsequently picked up.
FIGS. 23 and 24 are schematic views illustrating a conventional solenoid valve 100. FIG. 23 shows a state in which the solenoid valve 100 is open, and FIG. 24 shows a state in which the solenoid valve 100 is closed.
In general, the solenoid valve 100 comprises a coil 104 for axially moving a substantially cylindrical plunger 102, a chamber 106 (shown only in FIG. 23) containing the plunger 102, and two holes 108a and 109a formed in the bottom of the chamber 106, through which two pipes 108 and 109 are connected to the chamber 106. When using the solenoid valve 100 in the apparatus disclosed in the above-mentioned US patent, the two pipes 108 and 109 are connected to respective suction nozzles and vacuum tanks.
When opening the solenoid valve 100, the coil 104 is excited to pull the plunger 102 out of the chamber 106 to make the two holes 108a and 109a communicate with each other via the chamber 106. In contrast, when closing the solenoid valve 100, the excitation of the coil 104 is interrupted to thereby push the plunger 102 into the chamber 106 and block the two holes 108a and 109a, thereby blocking a fluid path 110 that connects the pipes 108 and 109.
The solenoid valve 100 is opened and closed by axially moving the plunger 102. Therefore, great inertia occurs when opening/closing the solenoid valve 100. In particular, when the diameter of the pipes 108 and 109 connected to the solenoid valve 100 is increased to increase the flow of air, it is necessary to also increase the diameter of the plunger 102 for blocking the holes 108a and 109a. In this case, greater inertia will occur accordingly.
Further, when opening the solenoid valve 100, much time is required until the pressure in the chamber 106 reaches a preset value after the coil 104 is excited to move the plunger 102 and introduce air into the chamber 106. Thus, the response rate of the solenoid valve 100, i.e., the rate of starting the circulation of air after the excitation of the coil 104, is low. Similarly, when closing the solenoid valve 100, the moving rate of the plunger 102 is low since the plunger 102 is pushed into the chamber 106 against air of the preset pressure. Namely, the response rate of the conventional solenoid valve 100 is low both when the coil 104 is excited, and when the excitation of the coil 104 is stopped.
This being so, if the solenoid valve 100 is used between the suction nozzle and the vacuum tank employed in the sheet (mail matter) pickup apparatus disclosed in the above-mentioned US patent, the mail matter pickup rate will inevitably be low because of the low response rate of the solenoid valve 100 itself.
Further, if the solenoid valve 100 is used in the sheet (mail matter) pickup apparatus disclosed in the above-mentioned US patent, it will be difficult to attach, by suction force, a relatively large and heavy mail matter on the belt with holes. Namely, when the solenoid valve 100 assumes the open state shown in FIG. 23, it is necessary to circulate air through a fluid passage bent at several points. In this case, the passing resistance of the air is not small, and hence it is difficult to increase the flow rate. This makes it difficult to draw a relatively large amount of air through the suction nozzle, i.e., difficult to attach a relatively heavy mail matter on the belt by suction force.